sockets: change inet_connect() to support nonblock socket

sockets: change inet_connect() to support nonblock socket

Add a bool argument to inet_connect() to assign if set socket
to block/nonblock, and delete original argument 'socktype'
that is unused.
Add a new argument to inet_connect()/inet_connect_opts(),
to pass back connect error by error class.

Retry to connect when -EINTR is got. Connect's successful
for nonblock socket when following errors are got, user
should wait for connecting by select():
  -EINPROGRESS
  -EWOULDBLOCK (win32)
  -WSAEALREADY (win32)

Change nbd, vnc to use new interface.

Signed-off-by: Amos Kong <[email protected]>
Reviewed-by: Orit Wasserman <[email protected]>
Reviewed-by: Michael Roth <[email protected]>
Signed-off-by: Anthony Liguori <[email protected]>